Packet switches (or packet transmitting apparatus) are generally classified into two categories: a pizza-box type and a chassis type.
A chassis type packet switch is a packet switch to be used by inserting, as necessary, a card-shaped unit into a chassis varying in size from around 40 cm high to more than 200 cm high. A chassis type packet switch includes circuit units each providing a function to perform interfacing, switching, or controlling (which are called interface unit, switch fabric, and control unit, respectively, and they are also called simply as cards), and a desired function can be implemented by inserting/detaching the corresponding units (or cards).
The switch configuration within a packet switch may be classified into a centralized switch and a distributed switch. A centralized switch has a switch fabric for switching among multiple interface units. A distributed switch does not have a dedicated switch fabric since the interface units thereof are connected to each other in a mesh-like fashion.
Since a chassis type packet switch allows a larger capacity configuration, it may often be used as a core packet switch of a business organization, or a packet switch in a metro/backbone area for a service provider. Therefore, sufficient availability is required for a packet switch. For this reason, each of units constituting a packet switch is generally configured to be replaceable and to continuously operate without adversely affecting services even when a failure occurs, by preparing a spare unit in advance. Hereinafter, the preparation of a spare unit in advance is called “redundancy”.
FIGS. 13A, 13B, 13C are diagrams illustrating examples of redundancy methods for units. The redundancy methods for units constituting a packet switch can be roughly classified into “1+1 redundancy”, “1:1 redundancy” and “1:N redundancy”.
FIG. 13A is a diagram illustrating an example of a method for 1+1 redundancy. In the case of 1+1 redundancy, one spare unit is prepared for one in-use unit (or one active unit). In a steady state ((a) in FIG. 13A), a transmitter transmits, to the spare unit, information having the same content as the information held in the in-use unit, that is, information copied from the in-use unit. Therefore, the spare unit has the same information as that held in the in-use unit. A receiver selects one of the in-use unit and the spare unit, as a unit from which it will receive information. When a failure has occurred in the in-use unit ((b) in FIG. 13A), the receiver selects the spare unit and receives the information from the selected spare unit. That is, a transmitter copies information and transmits the copied information to both of the in-use unit and the spare unit while a receiver selects one of the in-use unit and the spare unit as a unit from which it will receive the information.
FIG. 13B is a diagram illustrating an example of a method for 1:1 redundancy. In the case of 1:1 redundancy, one spare unit is prepared for one in-use unit. In a steady state ((a) in FIG. 13B), a transmitter transmits information only to the in-use unit, and a receiver side receives the information from the unit in use. When a failure has occurred in the unit in use ((b) in FIG. 13B), the transmitter side changes the destination of information from the in-use unit to the spare unit, and the receiver receives the information from the spare unit. This is, a transmitter selects the destination of information while a receiver receives the information from the spare unit.
FIG. 13C is a diagram illustrating an example of a method for 1:N redundancy. In the case of 1:N redundancy, one spare unit is prepared for N in-use units. FIG. 13C illustrates a case where N=2. In a steady state ((a) in FIG. 13C), a transmitter transmits information to the corresponding in-use units, and a receiver receives the information from the corresponding unit. When a failure has occurred in one of the N in-use units ((b) in FIG. 13C), the spare unit takes over the task executed by the in-use unit in which the failure has occurred. The 1:1 redundancy can be regarded as a special case of the 1:N redundancy where N=1.